Primers are typically coated over metal substrates to provide at least some corrosion protection. They may be applied directly to the substrate, but quite often are applied over a previously treated surface. For example, U.S. Pat. No. 3,671,331 teaches the application of a weldable primer over a metal substrate first coated with special hexavalent-chromium-containing composition.
The primers can often be applied as mill finishes, such as directly in a coil coating operation. Subsequent metal operation can include stamping and drawing, subjecting the coating to a severe shear force. A test for primer adhesion subjected to shear force has been discussed, for example, in U.S. Pat. No. 3,954,510. Therein it is noted that quite good primer adhesion can be obtained, particularly from careful control in the make-up of the composition of a pre-treatment formulation.
Where the primer coating is a zinc dust primer, it has been proposed in U.S. Pat. No. 3,948,686 to lubricate the coating, prior to metal deformation, with a film composed of a resinous polymeric binder plus a lubricating substance such as a wax, fat, graphite or oil. The film is typically baked on. It would, however, be desirable to apply a lubricant film that, after service, could be easily removed.